m15 R4, purchase motherboard and battery

I would usually suggest parts-people.com for this but it looks like they have no stock so the next option would be ebay or Aliexpress. 

https://www.parts-people.com/index.php?action=category&id=151&subid=1493&refine=motherboard

You can use the part numbers in the link above to search other sites. It will depend on what CPU/GPU combo you are looking for.

It may not be cost effective to replace the board though and it may be better to invest that money on a new or refurbished laptop. Dell have an online outlet store with refurb laptops at reduced prices.